An ignition coil device in which an insulating resin material is vacuum-filled into the whole of a housing is disclosed as a stick-type ignition coil device in U.S. Pat. No. 6,469,608 (JP-2001-185430A). An axial cross-sectional view of an ignition coil device of the same type as the ignition coil device disclosed in the above patent document is shown in FIG. 9. As shown in this figure, an ignition coil device 100 has a center core 101, a secondary spool 102, a secondary coil 103, a primary spool 104, a primary coil 105, an outer peripheral core 106, a housing 107, and a high voltage tower 108.
The housing 107 is shaped like a cylinder. The center core 101 is shaped like a round bar and is arranged nearly in the radial center of the housing 107. The secondary spool 102 is cylindrical and is arranged on the outer peripheral side of the center core 101. The secondary coil 103 is wound around the outer peripheral surface of the secondary spool 102. The primary spool 104 is cylindrical and is arranged on the outer peripheral side of the secondary coil 103. The primary coil 105 is wound around the outer peripheral surface of the primary spool 104. The outer peripheral core 106 is shaped like a cylinder with a slit and is arranged on the outer peripheral side of the primary coil 105. The high voltage tower 108 covers the bottom end opening of the housing 107.
An epoxy resin 109 is filled from the top end opening of a housing 107 into the housing 107 and a high voltage tower 108 which are evacuated to a vacuum. Then, the epoxy resin 109 is cured in the spaces between the respective parts. The epoxy resin 109 ensures the insulation between the respective parts. Thus the epoxy resin 109 fixes the respective parts. However, the ignition coil device 100 has a large number of parts. For this reason, the ignition coil device 100 has a complicated structure and needs many assembling man-hours.
Moreover, a primary part of low voltage such as the primary coil 105 and a secondary part of high voltage such as the secondary coil 103 need to have a predetermined insulation dimension between them so as to prevent dielectric breakdown. However, in a method of manufacturing the ignition coil device 100, first, resin parts such as the secondary spool 102, the primary spool 104, the housing 107 and the high voltage tower 108 are molded separately, and then these molded parts are assembled. When they are molded, some of them cause molding defects such as shrinkage, warpage and twisting. In some cases, an unexpected percentage of shrinkage in molding and the deformation and wear of the mold cause dimensional errors. For this reason, in order to ensure a predetermined insulation dimension, it is necessary to incorporate these molding defects and dimensional errors into the dimensional tolerances of the respective resin parts and to set the dimensions and locations of the respective resin parts.
Here, the stick-type ignition coil device is directly mounted in a plug hole. For this reason, the outside diameter of the ignition coil device is regulated by the inside diameter of the plug hole. Thus, the outside diameter of the ignition coil device is preferably as small as possible. However, the outside diameter of the ignition coil device is inevitably enlarged by the integration of the dimensional tolerances of the respective resin parts.
Moreover, a high voltage transformer in which insulating resin material vacuum-filled into the spaces between the parts is integrally molded with a housing as disclosed in JP-A 7-230931. The application of the high voltage transformer can reduce the parts in number because the housing is integrally molded. Moreover, it is possible to remove the dimensional tolerances in a case where the housing is molded alone from the integration of the tolerances, that is, the integrated tolerances. However, the housing is comparatively simple in the construction of planes and has a small change in thickness. Thus, the housing resists causing molding defects and dimensional errors. The housing is a part for forming a cover of the ignition coil device and is not a part interposed between the primary windings and the secondary windings. Namely, the proportion of the dimensional tolerance of the housing to the integrated tolerances is small. For these reasons, the dimensional tolerance of the housing is essentially small. Therefore, it is difficult to reduce the outside diameter of the ignition coil device.
Moreover, JP-A 9-246070 and Japanese Utility Model 3026649 disclose an ignition coil device having no housing, that is, an ignition coil device whose outer peripheral core is exposed. In these ignition coil devices disclosed in these documents, insulating resin material is vacuum-filled into the whole inner peripheral portions of the outer peripheral core. According to these ignition coil devices, the parts can be reduced in number. Since the ignition coil devices are not provided with the housing, the dimensional tolerances of the housing can be removed from the integrated tolerances. However, the proportion of the dimensional tolerance of the housing to the integrated tolerances is small. The thickness of the housing itself is comparatively small. For this reason, even if the ignition coil device has no housing, it is difficult to reduce the outside diameter of the ignition coil device.